The invention relates to an electromechanical functional module. Such functional modules, e.g. in the form of piezoelectric transducer elements, are used primarily for structure systems, which with self-regulating mechanisms can adapt to changing environmental conditions and are referred to as adaptive structures or smart structures. In such adaptive structures, sensors and actuators in combination with suitable controllers are integrated into the structure. Thus, such a structure is able to detect external changes and to respond appropriately to them. In contrast with conventional structures having passive spring or damping elements, the adaptive components form an integral part of the structure. Disturbances, such as unwanted deformations or vibrations for example, can be counteracted directly at the point of origin.
Since the structures combine both support functions and actuatory or sensory functions, these structures can provide a greater potential for lightweight construction and applications associated with the aerospace technology. In addition, however, there are also diverse possibilities for applications in other industries, e.g. for the reduction of noise and vibrations, for contour deformation and stabilization (shape control) and for high-precision positioning.
Piezoceramic materials exploiting the piezoelectric effect or the inverse piezoelectric effect may advantageously be used as actuators and sensors, which are integrated into the structure. Due to their constitution, however, these piezoceramic materials are extremely fragile and, accordingly, break very easily. In particular, this disadvantage becomes clearly apparent in the use of thin, discoid piezoceramics or piezofilms having a thickness of approximately 0.2 millimeters (0.0078 inches). Accordingly, due to the fragility of the piezofilm, the piezofilm was conventionally enveloped for protection prior to the installation of a piezofilm into a functional module. This provides defined mechanical and electrical boundary conditions for the piezofilm. By this means, the handling of the piezofilm is considerably simplified. In these electromechanical functional modules, an electrical contact for the electrodes of the piezoceramic transducer and electrical connectors for the transducer are embedded in the functional module.
Such electronic functional modules can be integrated as expansion and flexure actuators or sensors into any structures or applied onto the latter. In addition, they can be produced in the form of complex geometries.
For the use of an electromechanical actuator, e.g. piezoceramic element, an electrical field is required, wherein the ceramic material is positioned inside the field. Said field is generated by electrodes, which are covering the transducer. Usually, the electrodes are thin metal layers covering the transducer completely at two opposite sides. In case of an electric current at the electrodes, an electrical field is formed between the electrodes. It is important, that there is a direct contact between the electrode and the piezoceramic transducer. Additional, dielectric or isolating layers are disturbing the electrical field. As a rule, the electrodes are therefore vacuum metallized or sputtered onto the transducer surface or burned into the transducer surface by means of electrical conducting paste. Characteristic for the conventional transducers is, that the layers are very thin, brittle and susceptible to cracks.
Thus, the contacts of the electrodes to the transducer are essential in order for a reliable use of piezoceramic transducers which are subject to high mechanical loads.
The use of piezoelectric transducers as both actuators and sensors is disclosed, for example, by U.S. Pat. No. 5,347,870, which issued to Dosch, et al. on Sep. 20, 1994.
U.S. Pat. No. 4,849,668, which issued to Crawley, et al. on Jul. 18, 1989, discloses the direct integration of piezoceramics in multilayered structures such as a carbon fiber laminate. Inner layers of the structures have cut-outs for accommodating piezoceramics. Insulating layers are provided between the piezoceramics. A disadvantage is that the piezoelectric actuators and/or sensors must have their contacts made and fabricated during the production of the structure. In addition, mechanical problems arise such as the fatigue resistance of the electric contacts, the electrical insulation of the current-carrying components and the risk of breakage of the fragile piezoceramic during production.
U.S. Pat. No. 5,485,053, which issued to Baz on Jan. 16, 1996, discloses a three-layered vibration and sound-damping structure in which a viscoelastic damping layer is arranged between two piezoelectric layers. One piezoelectric layer serves as vibration sensor while the other piezoelectric layer is used as an actuator for the compensation of the vibrations.
U.S. Pat. No. 5,378,974, which issued to Griffin on Jan. 3, 1995, discloses the use of piezoceramic actuators driven in opposite directions for a vibration-damping system. A corresponding system is also described in U.S. Pat. No. 5,315,203, which issued to Bicos on May 24, 1994 and discloses the electric field of one piezoelectric element being applied in the opposite direction to a second piezoelectric element. In this manner, an oppositely directed deformation is brought about without the need for other control mechanisms.
Furthermore, piezoelectric functional modules are known that can be built into composite structures as prefabricated compact elements. Thus, U.S. Pat. No. 4,876,776, which issued to Whatmore, et al. on Oct. 31, 1989, discloses the fitting of piezoelectric elements into a composite structure, the composite structure having recesses for accommodating the piezoelectric elements and being prefabricated before the installation of the piezoelectric elements.
U.S. Pat. No. 5,305,507, which issued to Dvorsky, et al. on Apr. 26, 1994, discloses the installation of a piezoelectric actuator or sensor in a nonconducting fiber composite material, such as a glass fiber or epoxide as examples. In this case, the piezoceramic elements are first completely wired and only then laminated into place.
U.S. Pat. No. 5,687,462, which issued to Lazarus et al. on Nov. 18, 1997, and U.S. Pat. No. 5,656,882, which issued to Johnson on Aug. 19, 1997 as well as PCT Application No. PCT/US95/01111 having an International Publication No. WO 95/20827, which was published on Aug. 3, 1995, discloses a piezoelectric functional module in which a piezoceramic is bonded into a polyimide film. Contact is made to the electrodes via thin applied strip conductors made from copper foil, which are likewise bonded between the polyimide films. Electric contact is made to the piezoelectric transducers via plugs, which are clipped onto the polyimide films.
PCT Application No. PCT/US95/01111, International Publication No. WO 95/20827, page 9, lines 26 et seq., further discloses the use of frame elements between the polyimide films for accommodating the piezoceramics, which also serve as spacers during fabrication. The frame elements are made from a relatively highly compressible material, such as a non-cross-linked polymer, having a low modulus of elasticity.
In the known piezoelectric functional modules, making electric contact is particularly problematic. In cases of long operating periods, cracking may be observed in the strip conductors formed of thin copper foil at the junction between the piezoelectric transducer and the surrounding sheath. Due to the contact being made by a copper foil, the electrode of the piezoceramic is also only incompletely covered so that in the event of a breakage in the piezoceramic, the loss of active performance of the piezoceramic does occur.
In the integration of the conventional piezoelectric functional modules in fiber composite structures it is also disadvantageous that relatively many fibers have to be cut to make the electrical connections to the outside. This directly impairs the strength of the fiber composite structure.
In addition, the adhesion of the polyimide film in fiber composite structures is relatively poor so that the surfaces require expensive treatment. Polyimide films also absorb relatively high amounts of moisture so that there is the risk of electrical short-circuiting when piezoelectric functional modules are operating in a moist environment.
The present invention is directed to overcoming one or more of the problems set forth above.